Among the copper-dependent amine oxidases the characterization of the active site cofactor in lysyl oxidase is of particular interest, due to the biological importance of this enzyme in connective tissue proteins. It initiates the covalent cross-linking in collagen and elastin by oxidative deamination of peptidyl lysine or hydroxylysine. Studies have shown that abnormal expression of lysyl oxidase is associated with several pathologic conditions. Severe defects in connective tissues are associated with a deficiency of lysyl oxidase. Recently, lysyl oxidase was purified from calf aorta and was derivatized by 14C-phenylhydrazine. Proteolysis of derivated protein and characterization of labeled peptides by mass spectral and sequencing techniques will be used to obtain definitive structural proof regarding the nature of the active site cofactor and its mode of peptide attachment in bovine aorta lysyl oxidase. The use of mass spectrometry is critical in characterization of the novel linkages of the organic cofactor to the peptide backbone.